Networks based on optical fiber transmission systems provide a high bandwidth communications medium for relatively low cost. Uses of optical fiber communication systems are therefore expanding. For instance, proposals exist to implement integrated voice and data transmission systems using optical fibers throughout the public telephone network, so that each home has access to optical fiber (or "fiber-optic") transmission systems.
Fiber-optic transmission systems communicate data by means of optical bursts, or pulses of electromagnetic energy. The distance that these bursts propagate from remote stations to a head end attenuates the magnitude of the bursts as received at a head end receiver. These pulses are grouped into packets, one for each remote station transmitter, which together form a frame. Also, the packets come from any of a variety of sources in the system at any time. Thus, receivers must be adapted in these systems to handle not only variations in the magnitude of received signals and some unpredictable timing which causes tradeoffs in receiver design. It is preferred that the receiver be capable of handling a wide dynamic range of magnitude of the energy packets it receives, have high sensitivity, and require short packet preambles.
FIG. 1 illustrates one prior art receiver whereby an optical signal is detected by a photodiode and amplified by an amplifier 1 coupled to a capacitor 2 which is coupled to a comparator 3 having a second input coupled to a reference voltage V.sub.R. The capacitor is also coupled to the reference voltage V.sub.R through resistor 4. According to this receiver, a short resistance-capacitance (RC) time constant is desired for the capacitor 2 and resistor 4 so as to minimize a length of time for the comparator 3 to establish an optimum charge, e.g. about one half of peak charge, so that a correct threshold for detection of the digital data in the packet can be established quickly. The optimum capacitor charge is obtained by using a packet preamble having consecutive ON-OFF pulses which represents transmission overhead. A short RC time constant is also desirable to achieve a large receiver dynamic range. However, an RC time constant which is too short can result in the capacitor becoming excessively charged when the packet data contains a successive string of binary ONs or becoming excessively discharged when the packet data contains a successive string of binary OFFs, each of which can result in bit errors being made by the comparator which minimizes the sensitivity of the receiver.
Steensma, et al., "A FIFTY MB/S FIBER OPTIC PACKETIZED VOICE AND DATA BUS USING RANDOM MULTIPLE ACCESS", published by ITT Defense Communications Division, Sep. 17, 1980 recognizes the need for a receiver having a large dynamic range. According to Steensma, et al., the incoming data is supplied to a detector having a first signal path, including a fifty nanosecond delay connected to one input of a comparator, and having a second signal path connected to a dynamic clamp which extracts the DC level or "pedestal" from the input signal to correct for changes in the magnitude of the received signal. The DC level is connected to the second input of the comparator to set the proper slicing level for digital signal regeneration. See pp. 33-38 and FIG. 13 of the Steensma, et al. publication.
The Steensma, et al. receiver works well for systems having very high signal levels. However, it is very inefficient at low signal levels. This arises because the peak detector used in its "dynamic clamp" is based on "hot-carrier diodes" or Shottky barrier diodes having a threshold of at least 200 millivolts each. Therefore, signals below the threshold of these diodes cannot be efficiently detected. Furthermore, the Steensma, et al. system has problems with temperature stability because of the diodes in the peak detector.
Accordingly, it is desirable to provide a protocol and a receiver for such protocol which is capable of supporting a wide dynamic range covering small signal levels (e.g. highly sensitive), which requires short packet preambles, which allows for data to have consecutive ON and OFF pulses, and which is suitable for use in large scale fiber optic communication systems. Also, it is desirable to provide a receiver and protocol capable of optimizing reception of data for asynchronous fiber-optic systems.